The present invention relates to data storage units, and especially to data migration, and, in particular, to a method and a storage unit to control hierarchical management of data migration to maintain balanced performance.
In a conventional hierarchical control system, such as disclosed in JP-A-9-44381, a controller executes migration of frequently accessed data to a storage unit having a high access speed and transfers infrequently accessed data to a storage unit having a low access speed, based on access frequency of data managed inside the controller.